Yes you can check in any time after the computers reset about 6:30am. No guarantee room will be ready until closer to officail check in time of 3pm. You can use all the resort amenities, just not your room. FYI you can also use the resort until midnight on the day you check out, just not your room after check out.
We've checked in at CR as early as 11am and the room was ready and we've checked in at 4pm and the room wasn't ready. There's no telling, but you can store your stuff with Bell Services and hit the pool, go the parks or DTD, no problem
If you know you're going to be physically at the hotel early I strongly recommend using 'online checkin' and indicating that.
We've had great luck / pixie dust with checking in early and having a room ready for us (then again we do generally travel during the 'off' season). We've gotten a room really really early in the morning. I always think online checkin gives them a heads up as to when to expect us - and I think a lot of times they just assign us to a room that isn't being used the night before (which rocks!)